1. Field of the Invention
The present invention relates to a method of introducing impurities into crystalline films of nitride-based compound semiconductor structures, and more particularly to a method of preparing high brightness visible and ultraviolet light-emitting devices featuring a planar top surface.
2. Description of Prior Art
Blue and green light-emitting diodes (LEDs) based on nitride semiconductors (e.g., gallium nitride (GaN), aluminum nitride (AlN), and indium nitride (InN)) have reached brightness levels which are comparable to traditional red and orange LEDs based on arsenide and phosphide semiconductor technologies. Brightness levels now exceed one candela in the blue, and have reached ten candelas in the green. Brightness is becoming high enough that soon these nitride LEDs will find general use throughout the commercial sector, for applications such as indicator lights, large flat panel displays, and traffic signals. Furthermore, purple diode lasers based on gallium nitride have recently been demonstrated which could provide four times the storage density now available for optical disks, which could enable secure satellite communications links to be created, and which could be used for medical diagnoses and therapies. However, techniques for fabricating nitride LEDs rely on complex technology for device fabrication which results in costs that are much higher than those encountered with the manufacture of standard red and orange LEDs, and laser diodes.
Most nitride semiconductor devices are prepared by metal-organic chemical vapor deposition (MOCVD). A series of thin films with different nitride compositions, and which contain minor concentrations of impurities (dopants) for controlling the electric current and the wavelength of the emitted light, are grown sequentially on a heterogeneous sapphire substrate. It is necessary to make two electrical contacts to any light-emitting or laser diode device, one to supply electrons to the n-type region, and the other to remove electrons from the p-type region. It is always necessary to grow the n-type region first, starting at the interface with the substrate, and the p-type region last, at the top surface. Unfortunately, sapphire is an electrical insulator, and thus a bottom contact to the substrate is not possible. Reactive ion etching has been used to bore a hole through the p-type layers and any possible undoped (intrinsic) layers to allow the placement of a metal contact to the top surface of the rear n-type region. This approach is costly and creates a non-planar surface. A metal film contact must be selectively deposited into the hole, without making contact with the sidewalls of the hole, and a wire must be bonded to this metal pad. A second metal film must be evaporated over the remainder of the top surface to form the contact to the p-type layer. The electron current must then flow laterally from the lower contact to the n-type rear layer and spread uniformly under the entire device before turning vertically to travel up through the active region to the top metal contact. If the current doesn't first spread uniformly throughout the entire n-type layer, then the current density passing up through the active region will not be uniform, and not all of the volume of the material will generate the same brightness.
This problem can be seen by referring to U.S. Pat. No. 5,321,713, dated Jun. 14, 1994 (Khan et al) where FIGS. 2 and 3 shows GaN-based laser diodes which feature a second contact to the rear n-type layer placed down in an etched hole. In U.S. Pat. No. 5,408,120, dated Apr. 18, 1995, Manabe et al. show an aluminum metal plug which has been used to fill the hole that was etched in a GaN LED to form a contact to the rear n-type layer (FIG. 1). In U.S. Pat. No. 5,563,422, dated Oct. 8, 1996, Nakamura et al. describe a method of making electrical contacts to a GaN light-emitting device on two different levels, as shown in FIG. 1 therein. In U.S. Pat. No. 5,578,839, dated Nov. 26, 1996, Nakamura et al. again show in FIGS. 1, 11, 12 the GaN-based light-emitting structure where metal electrode 24 is deposited on a ledge after etching. Koide et al in U.S. Pat. No. 5,587,593, dated Dec. 24, 1996, show a method for etching a hole through the p-type layer 6 and light-emitting layer 5 down to the n-type layer 4 and filling the hole with nickel metal 8 to make the contact, as shown in FIG. 1. It is clear that many workers have come up with similar schemes involving etched holes to allow an electrical contact to be made to the rear n-type contact.
To allow for the presence of the non-conducting transparent sapphire substrate, nitride devices have been mounted substrate-down on a chip carrier, with wire bonds attached at two different levels on the top, complicating the assembly process. Two wires must be bonded at two different levels at or near the top surface of the device. Heat must be removed through the sapphire substrate which offers very poor thermal conduction. This heating problem is especially detrimental to laser performance and longevity. For an LED, light must be emitted through the top surface metal contact, reducing brightness. Also, one half of the light travels down through the sapphire substrate and is lost by absorption in the chip carrier. (See especially U.S. Pat. No. 5,563,422, FIG. 1.)
In contrast, most silicon devices are formed by a planar process which consists of the selective introduction of dopant atoms into small areas of the silicon wafer from the top surface of the wafer in order to form regions of n-type and p-type material. This technology is called planar because fabrication is accomplished by processes carried out from one surface plane. The crucial advantage of planar technology is that each portion of the fabrication process is applied to every device on the entire wafer at the same time. Attachment of leads is simplified when all contact pads are on the same plane, rather than located in holes below the surface. Gallium-arsenide (GaAs)-based LEDs and lasers are grown on a conducting substrate, which substrate forms one of the contacts after metallization. This contact can be fashioned as a mirror to reflect light back out through the top surface, doubling the brightness. Only one contact is needed on the top surface, which remains planar. For a GaAs laser, the chip is always flipped over and the p-type top growth surface is soldered to a heat sink, so that it is not necessary for the heat that is generated in the device to be extracted slowly through a substrate which is a poor thermal conductor.
In a recently reported technique, n-type conductivity was reported after implants of Si.sup.+ and p-type conductivity after implants of Mg.sup.+ P.sup.+ in nominally undoped samples of GaN films (Pearton et al., (1995) Appl. Phys. Lett., Vol 67, pp. 1435-1447). They found that Mg.sup.+ implantation (dose=5.times.10.sup.14 cm-.sup.2, 180 keV) did not produce p-type conductivity; but Mg.sup.+ P.sup.+ co-implantation produced p-type conductivity when samples were annealed at 1050-1100.degree. C. Also, implantation of Si.sup.+ gave a two order of magnitude increase in conductivity after annealing above 1000.degree. C.
.sup.40 Ca or .sup.16 O were implanted each with a dose of 5.times.10.sup.14 cm.sup.-2 at energies of 180 and 70 keV, respectively, to place the ion peak 100 nm below the surface of two separate GaN films (Zolper et al., (1996, Appl. Phys. Lett., Vol, 68, pp. 1945-1947). Samples were annealed for 10-15 sec in flowing N.sub.2 between 900 and 1150.degree. C. Ca became activated at 1100.degree. C. even without any co-implant specie and gave indications of p-type conductivity. Implanted O ions were also found to be activated, giving n-type conductivity. A GaN junction field-effect transistor that was created by ion implantation of Ca has been reported (Zolper et al., (1996), Appl. Phys. Lett., Vol 68, pp. 2273 2275).